The present invention relates to a spectrophotometer.
Twin-beam spectrophotometers evaluate a sample in terms of spectral transmittance, spectral reflectance, and other optical characteristics, by splitting the beam emitted from a light source into a sample beam and a reference beam and using the ratio of the sample beam signal S and reference beam signal R obtained by converting photodetector output signals by use of an amplifier and an A/D converter.
During wavelength spectral measurement using such a spectrophotometer, changes in wavelength, λ, change the levels of signals S and R according to the particular geometric value of E(λ).M(λ).D(λ) which consists of values such as the energy, E(λ), of the light source, the efficiency, M(λ), of the optical system, and the sensitivity, D(λ), of the photodetector. The accuracy of the data measurements obtained from digital signals depends on the number of digits in the particular digital signal, and the rate of change due to wavelength is, for example, in visible and ultraviolet radiation regions, several tens to several hundreds of times as great. Thus, in the wavelength regions that the detection system cannot attain high sensitivity, the accuracy of the signal ratio with respect to the wavelengths at which the detection system can attain high sensitivity decreases to several tenths to several hundredths of times as great. Accordingly, measurement accuracy significantly decreases in low-sensitivity wavelength regions.
A means effective for avoiding such decreases in measurement accuracy according to the particular wavelength of the signal values is by adopting the so-called “differential feedback” method, in which, each time data is acquired, the signal threshold value that establishes the condition of Atar=(Amax+Amin)/2 . . . where A is signal R or S, whichever is the greater in level . . . is to be assigned and when A≦Amin or Amax≦A, the sensitivity of the photodetector is to be adjusted so that signal A always satisfies Amin≦A≦Amax. By providing such adjustment each time a data acquisition operation is to be performed, it is possible to obtain almost equal signal accuracy at all times, irrespective of the differences in sensitivity according to wavelength region.
Photomultipliers (also referred to as photomultiplier tubes) are most commonly used as the photodetectors for measurement in visible and ultraviolet radiation regions, and since the sensitivities of photomultipliers change according to the value of the voltage applied between the cathode and anode of the photomultiplier, its detection sensitivity can be adjusted by varying the value of the voltage to be applied. Output signals from the photomultiplier, however, vary with the product of the sixth to tenth power of (incident light intensity +applied voltage), and thus in a high-sensitivity wavelength region and in a low-sensitivity wavelength region, even if exactly the same correction voltage is applied, a difference in the magnitude of the signal value adjusted will arise from the difference in the original applied voltage. In the photomultiplier, although the magnitude of the output current is taken out as a signal level, if the output current increases too excessively, the output current itself will saturate and the resulting disturbance in the proportionality mentioned above will reduce measurement accuracy. For this reason, Atar is set to the vicinity of the upper limit of the range in which the output current from the photomultiplier does not saturate. In such a case, in order to avoid output saturation associated with voltage correction, the voltage to be applied when one voltage correction operation is to be performed is set to a relatively small value with a high-sensitivity wavelength as its reference.
In this method, Amin≦A≦Amax always holds when variations in wavelength with each data acquisition operation are sufficiently small in comparison to changes in sensitivity. However, when spectral measurement that requires high-speed wavelength shifting (changes the wavelength at high speed) takes place, changes in the sensitivity of the detection system due to changes in wavelength will increase according to the particular variation in data acquisition wavelength. The consequent decrease in the relative voltage correction value with respect to the particular change in detector sensitivity will reduce the number of digits in the digital signal value (Amin>>A) or saturate the output current of the photomultiplier (A>>Amax), thus resulting in equal measurement accuracy not being obtainable in the entire measuring wavelength region.
Since a wavelength region in which detection systems can attain high sensitivity must be selected as the basis for specifying the voltage correction value, the occurrence of the problem described above has been particularly significant in the wavelength regions that detection systems cannot attain high sensitivity.
Although this problem can be solved by conducting measurements at reduced wavelength shifting speeds, when a multitude of samples are to be measured or when the measuring wavelength range is too wide, reduction in the wavelength shifting speed takes a greater deal of time to perform one measuring operation. When a multitude of samples are to be measured, therefore, it takes a great deal of time to measure all samples accurately.
One method of solving the problem involved is by calculating the voltage correction value from the present voltage value by use of a function, as described in Japanese Application Patent Laid-Open Publication No. Hei-72037 (1993). According to this method, the voltage signal correction value calculated from the present voltage value will be incorporated during next data acquisition. When the wavelength shifting speed is increased, the difference, |(λ0−λ1)|, between the present wavelength, λ0, and the next data acquisition wavelength, λ1, will also increase in proportion to the speed. Therefore, this method poses the new problem that during spectral measurement in the wavelength band where the detection system abruptly changes in sensitivity, the amount of correction does not keep up with any changes in signal level or the amount of correction increases too significantly in itself.
During wavelength spectral measurement, the measuring wavelength will shift in one direction each time data is acquired. However, with the correction method where the voltage value for the previous data acquisition is to be incorporated into the present voltage value, accurate sensitivity correction is not possible.
To maintain constant spectral measurement accuracy, it is necessary that sample beam signal S or reference beam signal R, whichever is the greater in level, should be taken as signal A and that a signal value within a fixed range should always be obtained as the value of signal A. In order to achieve this, the voltage to be applied to the photomultiplier is controlled to provide differential feedback correction so that the value of signal A always stays within a fixed range. With the prior correction method, however, the energy of the light source, the optical system, and the wavelength characteristics at the sensitivity of the photodetector do not permit constant detector output to be maintained when the measurements that require high-speed wavelength shifting (changing) are conducted, because the relative amount of correction with respect to a change in sensitivity will decrease, especially in the wavelength regions that the sensitivity of the detection system is low. The appearance of such a problem has been significant, especially in the wavelength regions that detection systems cannot attain high sensitivity.
Although such a problem can be solved by reducing the wavelength shifting speed, when the measuring wavelength range is too wide, reduction in the wavelength shifting speed takes too much time to perform measurements.
This problem also applies to a single-beam spectrophotometer that uses a single beam to perform spectral measurements.